The Complement System: An Overview of Its Cascade, Pathways, Regulation and Functions

THE COMPLEMENT SYSTEM
• Important effector in both innate and
acquired immunity
• Over 30 circulating and membrane-bound
proteins (synthesized in liver and other
cells- immune and epithelial)
• Acts as a cascade (one event must occur before
another takes place)
Dr Reena Kulshrestha, M.Sc, PhD.

Cascade:
• Many of the components are enzymes
that become activated when cleaved into
two peptides
• The larger peptide (joins the cascade)
binds to the immune complex and
becomes a functional part of it
• The smaller peptide diffuses away and
can become an inflammatory mediator
(binds to a receptor)

Complement is an acute phase protien.
It belongs to triggered enzyme cascade
system.
Every step has its own control
mechanism.
 It is present in sera of all mammals.

It is non-specific i.e. Complement of one species
can react with antibody of other species.
It constitute 5 % of normal serum protein & the
level rises during acute phase inflammation and is
not increased by immunization.
Fixation of complement is not influenced by nature
of antigen but class of immunoglobulin.
The complement binds on the fc part of
immunoglobulin CH2 (constant heavy domain) of
IgG & 4 of IgM.
Complement does not bind to free antigen &
antibody.
All classes of immunoglobulin do not fix
complement only IgM , IgG3 , IgG1 & IgG2 fix in
the respective order.
IgG4 , IgA , IgD & IgE do not fix complement.

Four important functions:
• Lysis
• Opsonization
• Activation of inflammatory
response
• Clearance of immune
complexes

Three pathways: Classical, Alternative, & Lectin
Final steps identical in all 3 pathways
1.Classical - Initiated by formation of an Ag- Ab
complex
2. Alternative - Antibody-independent
- Part of innate immunity
- Initiated by foreign cell
surfaces
3. Lectin - Initiated by host proteins binding
microbial surfaces

Classical pathway
• Classical was discovered first (but
actually evolved later)
• Initiated by:
-formation of a soluble Ag-Ab complex
-binding of antibody to a target such as
a bacterial cell
• Only certain antibodies can initiate this
(IgM, some classes of IgG)

CLASSICAL
PATHWAY-

E+A
EACl Ca++
EAC1
C4
C4a C4b
EAC14b
Mg++

C2
C2a C2b
It has kinin like
activity and increases
vascular permeability.
EAC14b2a (C3 convertase)
C3
C3a C3b
Chemotatic
and
anaphylatoxic
properties
EAC14b2a3b (C5 convertase)

C5
C5a C5b
Anaphylatoxic
and
chemotactic
activity
C567
C6
C7
Binds to cell
membrane and
prepares the cells
for lysis by C8 and
C9.
C8
C9
C14b2a3b5b6789
Cell damage or lysis.Dr Reena Kulshrestha, M.Sc, PhD.

Alternative pathway
• Four components: C3, factor B, factor D,
properdin
• Triggering substances may be pathogens
or nonpathogens :
bacterial cell wall components,
fungi, viruses, parasites
immune complexes, RBCs, polymers

ALTERNATIVE
PATHWAY-

Activator e.g.
endotoxin
C3b (bound)
factor BMg++
C3bB
C3bBb
Ba factor D
+
C3b in
circulation
Free C3b
inactivated
by factors
H and I.
(C3 convertase)Dr Reena Kulshrestha, M.Sc, PhD.

C3
C3a C3b
Chemotactic
and
anaphylatoxic
C3bBb3b (C5 convertase)
C5
C5a C5b
C6
C7Dr Reena Kulshrestha, M.Sc, PhD.

C567
Binds to cell
membrane and
prepares the cells for
lysis by C8 and C9
It also sensitises
bystander cells to
make them
susceptible to
lysis.
C8
C9
C3bBb3b5b6789
Cell damage or lysis.

Differences Between classical and alternative pathway
1) Activators
Antigen antibody complex Bacterial endotoxins
Retroviruses IgA & IgD
C Reactive protein Cobra Venom
DNA Nephritic Factor
Trypsin like Enzyme Zymogene
2) Starting component
C1q C3
C3 activation by PZ(Properdin zymogene complex)
Mg++ Proteinase enzyme
C3a
C3
C3b
3) Attachment to Fc No attachment.
C1q has 6 combining sites.
Recognizes Fc of IgM & IgG.
4) Components
11 components C3 factors BDPH ,5,6,7,8,9
C1q, r,s,2,3,4,5,6,7,8,9
5) C3 convertase
________ _ ____
C1 4b 2b C3b Bb
Dr Reena Kulshrestha, M.Sc,
PhD.

Lectin pathway
• Lectin is a protein that binds to carbohydrate
• MBL (mannose-binding lectin) binds to
mannose on many bacterial cells
- MBL is produced by liver in acute-phase
inflammatory reactions
• Once MBL binds to target cell, 2 serine
proteases (MASP-1, MASP-2) bind
• Acts like C1

Lectin pathway (mannose binding pathway)
Macrophages ingenst virus bacteria other foreign material
Release chemicals which stimulates
Liver cells
Acute phase proteins released like ( Mannose binding protein)
Binds to pathogens
Opsonisation Binds to lectin Binds to MASP
Phagocytosis Alternative pathway Mannose associated serine esteraseDr Reena Kulshrestha, M.Sc,
PhD.

The Complement CascadeThe Complement Cascade
C5
C5a
Ba Properidin
D
C3 P C3a
B
The Alternative Pathway
C3b C3bB C3bBb C3bBbP C3bBb3
b
B
C3C3iC3iB
Ba
C3iBb
C3
C3a
The Tick-over Activation
C5b
C4
C4a
C2 C2b C3 C3a
The Classical Pathway
Antibody +
C1qrs
C4b C4b2a C4b2a3
b

Regulation of complement system
• Because it is nonspecific, several
regulatory mechanisms are involved
(otherwise there would be a lot of
“collateral damage”)
• Many components are very labile
• Many regulatory proteins block activity
through binding to target

REGULATON OF COMPLEMENT
ACTIVATOR
[A] INHIBITOR
(1). C1 ESTERASE
* INHIBIT CLR,CLS BY BINDING TO ACTIVE SITE.
*MURAMINOGLYCOPROTEIN
*HEAT LABILE
*PREVENTS NORMAL CASCADE
*CHECKS AUTOCATALYTIC PROLONGATION

ACTIVATOR
(2).S PROTIEN
*BINDS TO C567
*MODULATES CYTOLYTIC
ACTION OF MEMBRANE
ATTATCH COMPLEX
[B] INACTIVATOR
(1) Fac1
*SERUM BETA GLOBULIN
*C3B,C4B ARE CLEAVED & IN
ACTIVATED

ACTIVATOR
*HOMEOSTATIC CONTROL OF C3 ACTIVATION.
*ENDOPEPTIDASE WHICH CLEAVES C3b & C4b.
(2) FACTOR H
*REGULATES ALTERNATIVE PATH WAY BY
PREVENTION FACTOR B TO BOUND TO C3
*BETA GLOBULIN FACTOR
*ACTS ALONG WITH FACTOR 1 MODULATIG C3
ACTIVATION.

ACTIVATOR
(3) ANAPHYLATOXIN INACTIVATORS
*ENZYMATICALLY DEGRADES C3a,C4a,C5a
(4) C4 BINDING PROTEIN
*CONTROLS ACTIVITY OF ALL BOUND C4b
*BINDS TIGHTLY TO C4b & ENHANCES C4b
DEGRADATION.

REGULATION OF COMPLEMENT
ACTIVATOR
(5) FACTOR P OR PROPERDIN
*BINDS TO A STABILIZER THE ALTERNATIVE PATH
WAY C3 CONVERTASE
*ACTIVATED PROPERDIN BINDS DIRECTLY TO BOUND
& UNBOUND C3b
*STABILIZE C3 & C5 CONVERTASE

ACTIVATOR
*INDUCE FURTHER ACTIVATION OF COMPLIMENT
COMPONENT INDIRECTLY BY EXTENDING THE
HALF LIFE OF C3bBb
(6)COBRA VENOM FACTOR (COVF)
*PROTEIN FOUND IN COBRA VENOM ACTIVATES
COMPLEMENT & LYSES ERYTHROCYTES

ACTIVATOR
*CAN COMBINE TO FACTOR B TO FORM COVF-Bb
COMPLEX i.e. EQUIVALENT TO C5 CONVERTASE
*VENOM IS RESISTANT TO ACTION OF
INACTIVATORS OF ALTERNATIVE PATH WAY
FACTOR H & I

BIOLOGICAL EFFECTS OF
COMPLEMENT SYSTEM
(1) INFLAMMATORY RESPONSE : C3a & C5a
*ANAPHYLATOXINS
*RELEASES HISTAMINE & OTHER MEDIATOR BY
DEGRANULATION OF MAST CELLS
*CHEMOTATICE-C567
*INCREASE VASCULAR PERMEABILITY,CONTRACTION
OF SMOOTH MUSCLE,VASODILATION,COLLECTION
OF INFLAMATORY CELLS,INCREASE IN PHAGOCYTIC
ACTIVITY

COMPLEMENT SYSTEM
(2)HYPER SENSITIVITY:TYPE II & TYPE III
(3) ENDOTOXIC SHOCK:
ALTERNATIVE PATH WAY,EXCESSIVE C3
ACTIVATION,PLATELET ADERENCE
(4) COAGULATION C3
*LYSES PROTHROMBIN
*THROMBIN RELEASES LARGE AMOUNT OF PLATELET
FACTOR

COMPLEMENT SYSTEM
DISSEMINATED INTRAVASCULOR COAGULATION &
THROMBOCYTOPENIA
(5) IMMUNE ADHERENCE :
*C3 & C4
*ANTIGEN ANTIBODY COMPLEXES + C
ADHERE TO ERYTHROCYTES OR PLATELETS
ENHANCED PHAGOCYTOSIS
*PHAGOCYTIC CELLS HAVE RECEPTORS FOR C3b

COMPLEMENT SYSTEM
(6) OPSONIZATION : CR1,CR2,CR9,CR4,CCq ON
MACROPHGES,NEUTROPHILL,MONOCYTES ETC.CR2
PRESENT OF B CELL
(7) AUTO IMMUNE DISEASES
E.G.-SLE & ANGIONEUROTIC OEDEMA
-DUE TO DEFICIENCY OF DOME COMPLEMENT FACTORS

COMPLEMENT SYSTEM
(8) SUSCEPTIBILITY
GRAM NEGATIVE BACTERIA – LYSIS
GRAM POSITIVE BACTERIA- WITHOUT LYSIS
(9) C3 & C6 PARTICIPATE IN COAGULATION PROCESS

Regulation of the ComplementRegulation of the Complement
CascadeCascade
 Short half-time ofShort half-time of
 C3bC3b
 C3bBbC3bBb
 C5bC5b
 C1 inhibitorC1 inhibitor
 Inhibits the C1s activityInhibits the C1s activity
 Protein S in SerumProtein S in Serum
 Binds to C5b67Binds to C5b67
→ Inhibits Formation of the Membrane AttackInhibits Formation of the Membrane Attack
ComplexComplex

 HRF or CD59HRF or CD59
 Bind to C8Bind to C8
 Inhibits C9 bindingInhibits C9 binding
 Factor HFactor H
 Binds to C3bBinds to C3b
→ Facilitates binding of Factor IFacilitates binding of Factor I
→ cleaves C3b to inactive iC3bcleaves C3b to inactive iC3b
→ cleaves C4b to inactive fragmentscleaves C4b to inactive fragments
 Decay Accelerating FactorDecay Accelerating Factor
 Increased dissociation ofIncreased dissociation of
C3 convertase (bothC3 convertase (bothDr Reena Kulshrestha, M.Sc, PhD.

Complement ActivationComplement Activation
GeneralGeneral
 Hydrophobic surfacesHydrophobic surfaces
 OxidesOxides
 Strong binding of C3(b) to nucleophilicStrong binding of C3(b) to nucleophilic
groups (-NH2, -OH)groups (-NH2, -OH)
 Higher absorption of C3 to crystalline TiOHigher absorption of C3 to crystalline TiO22
than to amorphousthan to amorphous
 Kallikrein directly activates C5Kallikrein directly activates C5
 Plasmin directly activates C5Plasmin directly activates C5

 Classical PathwayClassical Pathway
 Antibodies IgM, IgG1, IgG2, IgG3Antibodies IgM, IgG1, IgG2, IgG3
 Lectin via the mannan binding proteinLectin via the mannan binding protein
(MBP) “Lectin Pathway”(MBP) “Lectin Pathway”
 Hageman Factor (F XIIa)Hageman Factor (F XIIa)
 Rough surfacesRough surfaces
 C-reactive protein (CRP)C-reactive protein (CRP)
 (Zirkonium, transiently)(Zirkonium, transiently)

Alternative PathwayAlternative Pathway
 PE debrisPE debris
 Acetylated chilosanAcetylated chilosan

Biological effects of complement
activation
• Complement fragments must bind to
complement receptors
expressed by various cells

AnaphylatoxinsAnaphylatoxins
Fragments C5a, C3a, C4aFragments C5a, C3a, C4a
 Degranulation of PhagocytesDegranulation of Phagocytes
 Reactive oxygen speciesReactive oxygen species
 ProstaglandinsProstaglandins
 MonocytesMonocytes
→→ IL-1, IL-6IL-1, IL-6
 Mast cellsMast cells
→→ HistamineHistamine
 ChemotaxisChemotaxis
 Only C5aOnly C5a

Consequences in vitro
• Lysis of “innocent” neighbour cells
– Red blood cells
• Activation of phagocytic cells
– Release of reactive oxygen species
– Release of mediators

ConsequencesConsequences in vivoin vivo
 Factors of complement activation at revised hipFactors of complement activation at revised hip
implantsimplants

One single studyOne single study ((Tang L.Tang L. et al.et al. J Biomed Mater ResJ Biomed Mater Res 4141: 333-340 (1998)): 333-340 (1998))
Au-Mercaptoglycerol induces strong inflammatoryAu-Mercaptoglycerol induces strong inflammatory
response in control animals.response in control animals.
No reaction in Complement-depleted animals.No reaction in Complement-depleted animals.

 Amplifies humoral response
 Destroys invading bacteria and viruses
(lysis by MAC)
 Inflammatory response
 Opsonization of antigen (enhances
phagocytosis)
 Virus neutralization
 Clearance of immune complexes

Some bacteria can resist lysis
• Gram-positive bacteria
• Some microbes produce inactivating
enzymes
• Nucleated cells are harder to lyse
• Not particularly effective against tumor
cells (they can endocytose MAC and
repair damage)

Inflammation
• many of the released fragments help
develop an inflammatory response
• C3a, C4a, C5a- anaphylotoxins
bind to receptors on mast cells and
basophils; degranulation
(smooth muscle contraction; capillary
dilation; fluid influx)
• also play a role in blood cell chemotaxis

Viral neutralization
• Some viruses activate alternative or lectin
pathway
• Antibody-mediated (classical) pathway is
more common
• Causes aggregation of viruses; can’t infect
host cells; more vulnerable to phagocyte
• Enveloped viruses can be lysed

Consequences of complement deficiency
• Early components of classical pathway (C1,
C4, C2)- immune complex disease
• can’t generate C3b, which is needed
for solubilization
• Recurrent Staph and Strep infections
(can’t lyse bacteria but seem to control
infections)
• Early components of alternative pathway-
not as serious; tendency to infections
by NeisseriaDr Reena Kulshrestha, M.Sc, PhD.

•C3 deficiencies (can’t activate C5 and form
MAC)
•Recurrent severe bacterial infections
•MAC deficiencies- recurrent Neisseria
infections
•Regulatory protein deficiencies
1.edema
2.RBC lysis

Methods for Investigation
General/Common pathway
– Lysis of sheep red blood cells
– Solid phase methods (ELISA, RIA)
• Products: C3a, C5a, sC5b-9
• Consumption of C3
– Ellipsometry
Classical Pathway
– Measurement of C1qrs
– Measurement of C2b or C4b2a
Alternative Pathway
– Measurement of Ba or C3bBb
– Measurement of Properidin

CLASSICALCLASSICAL ALTERNATIVEALTERNATIVE LECTINLECTIN
ACTIVATINGACTIVATING
SUBS.SUBS.
IMMUNEIMMUNE
COMPLEXESCOMPLEXES
(IgG OR IgM)(IgG OR IgM)
LPS (bacterialLPS (bacterial
capsule)capsule)
IgAIgA
MannoseMannose
groups ongroups on
microbial cellmicrobial cell
RECOGNITIONRECOGNITION
UNITUNIT
C1q, C1r, C1sC1q, C1r, C1s C3, Factor B,C3, Factor B,
Factor DFactor D
MBP, MASP-1,MBP, MASP-1,
MASP-2MASP-2
C3C3
CONVERTASECONVERTASE
C4b2aC4b2a C3bBbC3bBb C4b2aC4b2a
C5C5
CONVERTASECONVERTASE
C4b2a3bC4b2a3b C3bBb3bC3bBb3b C4b2a3bC4b2a3b
MACMAC
C5b6789C5b6789
END RESULTEND RESULT
CELL LYSISCELL LYSIS

Biosynthesis of C
1) C1 - Intestinal epithelium
2) C2 C4 - Macrophages
3) C5 C8 - Spleen
4) C3 C6 C9 - Liver
5) C7 - Not known
6) Factors - Macrophages
B,D,P & I

Compliment deficiency and associated diseases
1) C1, C2, C3 &C4 disorders - Immune and rheumatic
2) C5, C6,C7,C8 disorders - Recurrent infection (Neisseria)
3) C1q disorders - Combined immunodeficiency states
4) C1r disorder - Many infections and lupus like symptoms
5) C1s disorder - Systemic Lupus Erythematossis
6) C4 disorder - Lupus like symptoms
7) C2 disorder - Increased susceptibility to infection
8) C3 disorder - Severe Pyogenic infections
9) C5 disorder - Recurrent infection of GIT
10) C6 C7 &C8 disorder - Disseminated gonococcal infections and recurrent
Mieningococcal Meningitis
11) C9 disorder - Not more susceptible to disease than other
individual in general populations.
12) C1 Inhibitors - Hereditary angioneurotic oedema
13) C3b inactivator factor 1
& factor D Properdin -Recurrent infection
PhD.

Key Facts
1. The complement system, a multi component triggered enzyme cascade, attracts
phagocytic cells to the microbes which engulf them.
2. Complement can be activated by classical and alternative pathways.
3. The amount of complement present in the serum cannot be increased by
immunization.
4. Complement participates in type II and type III Hypersensitivity.
5. Several serum compliment components are lowered in many auto immune
diseases such as systemic lupus erythematosus & Rheumatoid arthritis. They may,
therefore, be involved in the pathogenesis of auto immune diseases.
6. Complement mediates immunological membrane damage.
7. C fragments released during cascade reaction help in amplifying the inflammatory
response.
8. C3 and C4 mediate immune adherence.
9. C3 and C6 participate in Coagulation process
PhD.

Summary
The complement system comprises a group
of serum proteins which, when activated,
plays an important role in antigen
clearance.
The classical, alternative and lectin pathways
have been described.
Elaborate regulatory mechanisms are required
to prevent damage to normal cells.

The Complement System: An Overview of Its Cascade, Pathways, Regulation and Functions

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The Complement System: An Overview of Its Cascade, Pathways, Regulation and Functions